High-temperature high-strength aluminum alloys processed through the amorphous state

ABSTRACT

Aluminum alloys having improved strength at 300° C. characterized by formation from an intermediate amorphous state to a final fcc matrix hardened by optimal 25 nm-diameter Ll 2  precipitates with an interphase misfit less than about 4% in all three dimensions and Al 23 Ni 6 M 4  precipitates where M is one or more elements selected from the group consisting of Y and Yb. An appropriate melt of aluminum with selected transition metals (Co, Cu, Fe, Ni, Ti, Y) and Ll 2  stabilizers (Sc, Yb) in amounts of about 2 to 12 and 2 to 15 atomic percent, respectively, is processed to achieve an intermediate amorphous state to dissolve Ll 2 -forming components. The amorphous alloys are then thermo-mechanically devitrified to a final crystalline microstructure. The alloys have good ductility and a short-term tensile strength exceeding about 275 MPa (40 ksi) at 300° C., and are useful for applications such as high-temperature turbine engine components or aircraft structural components.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part utility application ofapplication Ser. No. 10/422,234 filed Apr. 24, 2003 entitled NanophasePrecipitation-Strengthened Al Alloys Processed Through the AmorphousState, which is based upon previously filed provisional applications:Ser. No. 60/375,940 filed Apr. 24, 2002 entitled “Amorphous metal alloycompositions” and Ser. No. 60/450,114 filed Feb. 25, 2003 entitled“Amorphous metal alloy compositions”, all of which are incorporated byreference and for which priority is claimed.

GOVERNMENT INTERESTS

Activities relating to the development of the subject matter of thisinvention were funded at least in part by United States Government, U.S.Army Aviation & Missile Command Contract No. DAAH01-02-C-R125, and thusmay be subject to license rights and other rights in the United States.

BACKGROUND OF THE INVENTION

In a principal aspect, the present invention relates to Al-based alloysprocessed through an amorphous state, preferably by means of a RapidSolidification Process (RSP) from molten alloy, and then devitrified toa primarily crystalline microscale fine grain structure bythermo-mechanical processing. To promote glass-forming ability, the Alalloys comprise selected transition metal (TM) and lanthanide rare earth(RE) elements. The final crystalline microstructure has a combination ofstable strength at or above about 300° C. and good ductility,characterized by optimal 25 nm-diameter Ll₂ precipitates in an fccmatrix with an interphase misfit typically less than about 4% in allthree dimensions, and rod-shaped Al₂₃Ni₆M₄ precipitates.

Improved strength at elevated temperatures has been a continuing goal inAl alloy development for more than three decades. Currently availablecommercial Al alloys, either manufactured with ingot or powderprocessing, are not capable of simultaneously achieving high strengthand high-temperature stability near 300° C.; such characteristics beingparticularly important in applications such as fan components in turbineengines. Precipitation hardening introduced by aging is a known methodto strengthen Al alloys. Conventional high-strength Al alloys incommercial applications employ Guinier-Preston zones and subsequentprecipitation at or below 250° C. Examples of Al alloys processed withrelatively high aging temperatures in commercial practice include alloy2618 (200° C. for 20 hours), 4032 (170-175° C. for 8-12 hours), and 2218(240° C. for 6 hours) [Metals Handbook-Properties and Selection:Nonferrous Alloys and Special-Purpose Materials, Volume 2, 10^(th)Edition, ASM International]. At the noted aging temperatures, thesealloys have an improved microstructure stability relative to othercommercial Al alloys. These Al alloys, when precipitation-hardened,usually possess a room temperature yield strength of about 600 MPa. (85ksi). However, at temperatures approaching 300° C., the precipitationhardening efficiency in these alloys quickly and significantly degradesas a result of precipitate coarsening and/or dissolution. Due to theunstable strengthening precipitate size distribution at such hightemperatures, the yield strength of currently available aluminum alloysat 300° C. is often only 10% of the yield strength at room temperature,and thus renders such alloys unsuitable for high-temperatureapplications above 150° C. For high-temperature turbine enginecomponents or aircraft structural components, a short-term tensilestrength exceeding about 275 MPa (40 ksi) at 300° C. is desired.

In order to achieve a combination of high strength and usablehigh-temperature properties in Al alloys, researchers have investigateda variety of intermetallic precipitation dispersions. The Al-based Ll₂phase is one of the best-known precipitates to achieve a goodcombination of high strength and high toughness of ambient temperatures.There are reportedly only seven elements stabilizing the Ll₂ phase: Er,Lu, Np, Sc, Tm, U, and Yb [Knipling, K. E. et al. Z. Metallkd97:246-265]. Since crystalline Al has very limited solubility for theseLl₂ stabilizers, it is difficult to produce a fine dispersion throughcrystalline solid-state heat treatments. Alternatively, with RSP fromthe liquid state, it is possible to either (1) directly produce a finecrystalline structure, or (2) produce partially amorphous Al alloys.Nonetheless, crystalline RSP Al alloys have not been able to meet thehigh-temperature strength requirements due to the difficulty ofproducing small, stable particles at adequate volume fraction. The focuson amorphous RSP Al alloys has been primarily on face centered cubic(fcc)-Al nanocrystals to enhance the ambient strength [Kim, Inoue,Masumoto, Mater Trans JIM 1990; 31: 747]. Upon devitrification, Alnanocrystals of up to 30% volume fraction can be dispersed within theamorphous matrix. However, this nanoscale ultra-fine grain stricture isundesirable because at high temperatures, typically ≧0.4-0.5 T_(m) whereT_(m) is the material's absolute melting temperature, the contributionof grain boundary strengthening is minimal and the refined grainstructures promote rapid diffusional creep. In addition, it has beenreported that ultra-fine grain sizes may be undesirable when,considering formability and fracture toughness [Hornbogen, Starke, Actametall. mater. 1993; 41: 1].

In sum, previous development on Al-based materials with high strength atelevated temperatures failed to meet the property objective of 275 MPaat 300° C.

SUMMARY OF THE INVENTION

The present invention is directed to a new class of Al alloyscharacterized by formation from an intermediate amorphous state to afinal fcc-Al matrix hardened by a combination of Ll₂ precipitates andAl₂₃Ni₆M₄ precipitates in order to establish the Al-based analogue ofNi-base superalloys and achieve high-temperature strength with usableductility.

An appropriate melt of Al with selected TM and RE is first processed toachieve an intermediate amorphous state to dissolve Ll₂-formingcomponents. The preferred method to achieve a primarily (above 70% involume) amorphous state is RSP from the molten alloy by processtechniques such as powder atomization, melt spinning, and spray casting.The RSP process should have a cooling rate of at least about 10³°C./sec, preferably at least 10⁴° C./sec. Other methods to achieveamorphous microstructure through a solid-state process, such asmechanical milling, may also be used. The intermediate amorphous alloysare then thermo-mechanically devitrified to a final primarily (above 70%in volume) fcc/Ll₂/Al₂₃Ni₆M₄ crystalline microstructure with at leastabout 70% fcc phase in volume, at least about 0.10% Ll₂ phase, and atleast about 10% Al₂₃Ni₆M₄ phase in volume.

The selection of alloying elements is based on (1) good glass-formingability with RSP, (2) long-term strength at or above 300° C., and (3)composition tolerance for a robust design. For glass-forming ability,elements with strong short-range ordering effects, and slow long-rangediffusing kinetics in molten Al are employed. For long-term strength ator above 300° C., the alloy of the present invention employs 25nm-diameter Ll₂ particles which are reported to provide optimal creepresistance [E. A. Marquis, Microstructural Evolution and StrengtheningMechanisms in Al—Sc and Al—Mg—Sc Alloys, Ph.D. thesis, NorthwesternUniversity, 2002.]. For a robust design, the present invention employsAl₂₃Ni₆M₄, where M is one or more elements selected from the groupconsisting of Y and Yb. When there is deficiency of the Ll₂-formers, theincoherent D0₁₁-Al₃Ni phase is expected to precipitate, leading to lowductility. Al₂₃Ni₆M₄ is more solute-rich that the Al₃X phase and willconsume less Al for a given amount of solute, giving rise to a higheramount of fcc matrix which in turn increases the ductility.

Thus, it is an object of the invention to provide a new class ofhigh-temperature high-strength Ll₂-phase strengthened Al alloysprocessed through the amorphous state, preferably with RSP, and thensubsequently devitrified with thermo-mechanical, processes.

A further object of the invention is to combine selected TM and RE toprovide good glass forming ability during RSP such as powder atomizationor melt spinning to form an amorphous Al alloy, dissolving theLl₂-stabilizers before the devitrification process.

Another object of the invention is to provide aluminum alloys withusable strength at or above about 300° C. by selecting Ll₂-stabilizerswhich reduce the interphase lattice misfit in all three dimensions topromote a finer dispersion.

Another object of the invention is to employ Al₂₃Ni₆M₄ precipitates toprovide composition tolerance and maintain reasonable alloy ductility.

These and a other objects, advantages and features will be set forth inthe detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWING

In the detailed description which follows, reference will be made to thedrawing comprised of the following figures:

FIG. 1 is an X-ray diffractogram of the alloy of Example 1 as melt-spunwith positions of fcc pure aluminum reflections indicating a fullyamorphous state;

FIG. 2 is an X-ray diffractogram of the alloy of Example 1 afterdevitrification at 550° C. for 24 hours, with positions of reflectionsof pure fcc Al, Al₃Yb, and Al₂₃Ni₆Yb₄ phases, indicating the desiredphases: fcc+Ll₂;

FIG. 3 is a Scanning Electron Microscope (SEM) secondary electron imageof devitrified alloy of Example 1 indicating phase constituentsfcc+Ll₂Al₂₃Ni₆Yb₄; and

FIG. 4 is an SEM secondary electron image of devitrified alloy ofExample 2 indicating phase constituents fcc+Ll₂+Al₂₃Ni₆Yb₄.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

General Summary

In general, the subject matter of the invention comprises an Al alloy incrystalline form having higher or greater strength particularly atelevated temperatures, i.e. greater than about 300° C. The Al alloy is;made by compounding a mixture of Al with selected TM and RE in amorphousstate followed by devitrification to a mixed crystalline statecomprising fcc, Ll₂, and Al₂₃Ni₆M₄ phases wherein the ratios of thecrystalline states are within certain preferred ranges. Preferably theresultant alloy has at least about 70% by volume fcc phase, at leastabout 10% by volume Ll₂ phase, and at least about 10% by volumeAl₂₃Ni₆M₄ phase where M is selected from the group consisting of Y, Yband a combination of Y and Yb with limited residual amorphous orquasi-crystalline phase material.

The choice of starting materials may vary, as may the compoundingprocesses, the glass formation processes and the devitrificationprocesses. In the amorphous state, there may be some crystallinematerial contained therein, but preferably no more than about 30% byvolume. The particle size of alloys passing through a fully or almostfully glassy state is much finer than that of alloys without passingthrough the glassy state or only passing through a partially glassystate with Ll₂ already present in the as-spun condition. Thus formingthe mixture in the amorphous intermediate state constitutes a veryimportant aspect of the invention.

The alloy materials, in addition to Al, include one or more TM taken orselected from the group of Cu, Ni, Co, Ti, Fe, Y, and Sc, and one ormore RE selected or taken from the group of Er, Tm, Yb, and Lu. TMmetals are utilized in the range of about 2 to 12 at %, and RE materialsare utilized in the range of about 2 to 15 at %.

The processes for mixing or forming the starting materials in theamorphous state are not necessarily limiting. Thus, it is contemplatedthat solid state processing, liquid or melt processing as well as gasphase processing may be utilized, though liquid phase processing ispreferred. The completeness of the amorphous state is at least about 70%by volume and preferably greater.

Development Technique

Precipitation-hardened Al alloys are difficult to develop for highstrength due to limited solubility of alloying elements. Al alloys withhigh fractions of precipitate that cannot be completely solution-treatedhave very coarse particles that tend to limit strength, corrosionresistance and toughness. In contrast, the Al alloys of the presentinvention exhibit high strength, good ductility, and high-temperaturestability at or above 300° C.

By carefully selecting an appropriate Al alloy composition, processingtechniques can achieve a fully amorphous state after rapid cooling.Furthermore, this glass can then be thermo-mechanically processed suchthat the glass devitrifies into a crystalline fcc matrix with nanophaseprecipitates. By passing through the glass state, the equilibriumsolidification that would produce coarse precipitates is avoided.Certain TM such as Fe, Co, Ni and Cu promote short-range ordering inliquid Al, which leads to low partial molar volume, low thermalexpansion, and high viscosity that are beneficial to glass-formingability. RE elements such as Ce, Gd, Yb, and Er with large atomic sizeexhibit low diffusivity in Al and thus retard crystal nucleation.Therefore, Al-TM-RE comprise a class of glass-forming system for Alalloys of the present invention.

The elements Er, Lu, Tm and Yb are reported as the only RELl₂-stabilizers. Among these four RE elements, Yb has the smallestlattice parameter and relatively low-cost. Er has the lowest cost. Toevaluate the effect on glass-forming ability of these alloyingadditions, a reduced glass transition temperature (T_(rg)) model wasdeveloped. In the Al-TM-RE system, this model predicts that Er has nobeneficial effect to T_(rg). As a consequence, alloys of the inventionutilize Yb as the preferred Ll₂-stabilizer rather than Er, Tm, and Lu.

Sc is the oily TM element that can form a stable Ll₂ with Al. Comparedto RE Ll₂ formers, Sc can form Ll₂ with a smaller lattice parameter,reducing the misfit between Ll₂ and Al matrix. However, Sc is by far themost expensive of the Ll₂-stabilizers and therefore embodiments of theinvention seek to limit. Sc as much as possible. Efforts have been madeto search for other TM to substitute for Sc. A preliminary requirementfor such substitution is solubility. Ti has a substantial solubility inAl₃Sc. In addition, Ti has the lowest diffusion coefficient in solid Alamong TMs. Adding Ti to Al₃Sc thus reduces the coarsening rate of Ll₂precipitates. Moreover, addition of Ti decreases the lattice parameterof Al₃(Sc,Ti) and hence minimizes the lattice misfit with Al. Thus,alloys of the invention incorporate Yb and Sc as base Ll₂ formers butare not limited to these elements. TM such as Ti, V, Zr, etc., whichwill result in low misfit and thus retard coarsening are considereduseful.

For a robust design, the present invention employs Al₂₃Ni₆M₄, where M isone or more elements selected from the group consisting of Y and Yb. Tointroduce both Al₂₃Ni₆M₄ and Ll₂ in the design, thermodynamicequilibrium calculations were performed using the thermodynamic databaseand calculation package Thermo-Calc® [Sundman, B. B. Jansson, and J. O.Andersson. 1985. Calphad 9: 153-190]. Thermodynamic calculations predictthat Y has certain solubility in Ll₂, which expands the Ll₂ latticespacing, increasing the misfit. Therefore, a design criterion should beset to limit the partitioning of Y in Ll₂. In addition, other phasessuch as Al₃Ni, Al₃Y and Al₉CO₂ should be avoided.

Al-base alloys will have good ductility when the amount of fcc is equalto or over about 70%. Thus, the total amount of Al₂₃Ni₆M₄ and Ll₂ isfixed to less than about 30%. At the desired phase constitution, Cocontent is set by [x_(Ni)+x_(Co)]/x_(Y)=6/4 because Co has a smallsolubility in Al₂₃Ni₆M₄ by substituting for Ni. After examining theeffect of Co addition based on thermodynamic calculations, an optimumwas found around 0.6 at % Co, at which partitioning of Y in Ll₂ isalmost zero. If Co addition is significantly more than 0.6 at %, Al₉CO₂and Al₃Y may precipitate.

Experimental Results

The present invention alloys, through, computational design ofmulti-component Al-TM-RE systems incorporate, desired processingproperties-glass forming ability and the desired microstructure—a finedispersion of Ll₂ after devitrification in the Al matrix.

Example 1

Prototypes of preferred embodiments can be made by arc-melting, meltspinning or wedge casting. Through melt spinning, ribbons ofAl-3.46Ni-2.78Y-0.72Co-0.42Yb-0.63Sc-0.42Zr-0.21Ti (at %) were made.Melt-spun ribbons are approximately 3-4 mm wide and 30-40μ in thickness.The ribbons were characterized using micro-hardness) x-ray diffraction,and SEM analysis. The x-ray diffraction pattern (FIG. 1) of the as-spunribbon indicates a partial amorphous microstructure withoutintermetallic precipitates. After devitrification at 550° C. for 24hours, x-ray diffraction (FIG. 2) shows precipitation of Al₂₃Ni₆Yb₄ andpeaks of Ll₂. It is noted that the peaks of Ll₂ are shifted compared toLl₂-Al₃Yb, indicating; decrease of lattice parameters due to dissolutionof Sc, Ti, and Zr in Al₃Yb. Such decrease of the Ll₂ lattice parameterwill reduce the misfit. FIG. 3 shows an SEM image of the devitrifiedspecimens confirming the phase constituents fcc+Ll₂+Al₂₃Ni₆Yb₄. Thematrix is fcc-Al, the large sized grey phase material is Al₂₃Ni₆Yb₄, andthe small white particles are Ll₂ phase particles. The Ll₂ particlesremain smaller than ˜50 nm in diameter, while the rod-shaped Al₂₃Ni₆Yb₄phase material is less than 1μ in length. The small Ll₂ particles willprovide optimal creep resistance at or above 300° C. and the Al₂₃Ni₆Yb₄material is present to avoid detrimental compounds and improve theductility at the high temperature.

Example 2

Ribbons of Al-3Ni-2.42Y-0.62Co-0.6Yb-0.6Sc-0.6Zr-0.6Ti (at %) were madeusing the protocol of Example 1. The ribbons were characterized usingmicro-hardness, x-ray diffraction, and SEM analysis. FIG. 4 shows an SEMimage of the devitrified specimens confirming the phase constituentsfcc+Ll₂+Al₂₃Ni₆Yb₄. The matrix is fcc-Al, the large sized grey phasematerial is Al₂₃Ni₆Yb₄, and the small white particles are Ll₂ phasematerial. The Ll₂ particles remain smaller than ˜50 nm in diameter,while the rod-shaped Al₂₃Ni₆Yb₄ phase material is less than 1μ inlength. The small Ll₂ particles will provide optimal creep resistance ator above 300° C. and the Al₂₃Ni₆Yb₄ material is present to avoiddetrimental compounds and improve the ductility.

Example 3

Scale-up processing of the alloy in Example 1 was engineered. Amorphouspowder produced by high-pressure He atomization can be used as a rawmaterial to produce an amorphous bulk by consolidation at hightemperatures. The amorphous alloy powder is produced by gas atomization,followed by sieving, precompaction, canning and sealing into a Cu tube,carried out in a well-controlled atmosphere with an oxide or moistureconcentration below 1 ppm. Powder of the alloy in Example 1 wassuccessfully atomized and extruded. The extrusion is a thermo-mechanicalprocess where the glass devitrifies into a crystalline fcc matrix withnanophase precipitates.

Example 4

Powder of the alloy in Example 2 was successfully atomized and extrudedusing the protocol of Example 3. The extrusion is a thermo-mechanicalprocess where the glass devitrifies into a crystalline fcc matrix withnanophase precipitates.

Variations of the described aluminum alloy as well as the process formanufacture thereof and the product created by the process arc availableto provide the expected functionality of high short-term and long-termstrength at temperatures above about 300° C. Thus the invention is to belimited only by the following claims and equivalents thereof.

1. An aluminum alloy characterized by high strength iii the temperaturerange greater than about 300° C. comprising, in combination: an alloymixture in primarily crystalline form having at least about 70% byvolume fcc phase, at least about 110% by volume Ll₂ precipitate phase,and at least about 10% by volume Al₂₃Ni₆M₄ precipitate phase where M isone or more elements selected from the group consisting of Y and Yb,said alloy consisting essentially of one or more transition metalsselected from the group consisting of about 2 to 12 atomic percent Co,Cu, Fe, Ni, Ti, and Y; and one or more elements comprising said Ll₂phase selected from the group consisting of about 2 to 15 atomic percentSc and Yb; optionally of transition metals selected from the groupconsisting of Cr, Li, Mn, V, and Zn, and the balance Al and incidentalelements and impurities; said Ll₂ phase in the form of a precipitateparticle dispersion having a particle diameter of less than about 80 nm.2. The alloy of claim 1 having a tensile yield strength of at leastabout 275 MPa at 300° C.
 3. An aluminum alloy characterized by highstrength at a temperature greater than about 300° C. made by a processcomprising the steps of: (a) formulating a melt comprised of Al; atleast one transition metal selected from the group consisting of Co, Cu,Fe, Ni, Ti and Y; at least one element selected from the Ll₂-stabilizingelement group consisting of Sc and Yb; optionally of transition metalsselected from the group consisting of Cr, Li, Mn, V, and Zn; (b)converting the melt to at least about 70% by volume amorphous material;and (c) devitrifying, at least in part, the amorphous material to amixture of Ll₂ crystalline rare earth precipitate phase material in aparticle dispersion wherein the particle size is less than about 80 nm,Al₂₃Ni₆M₄ precipitate phase where M is one or more elements selectedfrom the group consisting of Y and Yb, and fcc phase material.
 4. Thealloy product by the process of claim 3 wherein the transition metal isprovided in an amount of about 2 to 12 atomic percent.
 5. The alloyproduct by the process of claim 3 wherein the Ll₂-stabilizing element isprovided in an amount of about 2 to 10 atomic percent.
 6. The alloyproduct by the process of claim 3 wherein devitrifying the amorphousmaterial comprises forming at least about 70% by volume fcc phase. 7.The alloy product by the process of claim 3 wherein devitrifying theamorphous material comprises forming at least 10% by volume Ll₂ phase inpartial form.
 8. The alloy product by the process of claim 3 whereindevitrifying the amorphous material comprises forming at least 10% byvolume Al₂₃Ni₆M₄ phase in partial form, where M is one or more elementsselected; from the group consisting of Y and Yb.
 9. The alloy product bythe process of claim 3 wherein converting the melt to amorphous materialcomprises at least one step selected from the group consisting of gaspowder atomization, water powder atomization and melt spinning.
 10. Thealloy product by the process of claim 3 wherein, devitrificationcomprises at least one step selected from the group consisting of hotisostatic pressing, thermal aging, and extrusion.
 11. The product by theprocess of claim 3 wherein converting the melt comprises rapidsolidification processing.
 12. An aluminum alloy consisting essentiallyof about 2-12 atomic percent of at least one transition element selectedfrom the group consisting of Co, Cu, Fe, Ni, Ti, and Y; about 2-15atomic percent of at least one element selected from the groupconsisting, of Yb and Sc; optionally of transition metals selected fromthe group consisting of Cr, Li, Mn, V, and Zn; and the balance Al andincidental elements and impurities characterized by greater than about70% crystalline microstructure with a dispersion of Ll₂ phase particlesgreater than 10% by volume and Al₂₃Ni₆M₄ phase particles greater than10% by volume, where M is one or more elements selected from the groupconsisting of Y and Yb, in a matrix of greater than 70% by volume fccphase generated by a rapid solidification process from a substantiallyamorphous vitrified phase, said particle diameter of said Ll₂ particlesin the range of about 1.0 to 80 nm.
 13. The alloy of claim 12 whereinthe nominal particle diameter of the Ll₂ particles is about 25 nm. 14.The alloy of claim 12, consisting essentially of about 0.7 atomic % Co,3.5 atomic % Ni, and at least one element selected from the groupconsisting of Sc and Yb.
 15. The alloy of claim 12 consistingessentially of about 0.7 atomic % Co, about 3.5 atomic % Ni, and Sc, Ti,Y, Yb, and Zr, cumulatively in the range of about 4 to 8 atomic %.